Wireless communication systems use antennas to communicate signals. A wireless local area network (WLAN) is a type of wireless communication system that communicates information between nodes in a given area.
Wireless communication systems use transmitters to transmit signals.
Types of Signals
Narrowband and Wideband Signals
Most current wireless communication systems are narrowband signal systems. Narrowband signals have signal bandwidths ranging from tens of kilohertz (kHz) (e.g. 50 kHz) to hundreds of kilohertz (500 Khz). In contrast, wideband, or broadband, signals have signal bandwidths greater than 1 MHz.
802.11 and 802.11a
One type of wideband signal is the signal used in WLANs using the Institute of Electrical and Electronic Engineers (IEEE) 802.11 standard. The IEEE 802.11 standard (802.11) outlines Media Access Control (MAC) and Physical Layer (PHY) specifications for WLANs.
The IEEE 802.11a standard (802.11a) is a part of 802.11 and addresses communications in high data rate wideband packetized wireless communication systems, covering frequencies of operation between 5 GHz and 6 GHz. 802.11a uses orthogonal frequency-division multiplexing (OFDM) modulation, which allows communication to occur at very high data rates by transmitting data over multiple frequency bins over a wide frequency range. All the discussions herein applicable to 802.11a is also applicable to IEEE 802.11g. The IEEE 802.11g OFDM standard is the same as 802.11a, with the exception of operating in the 2.4 Ghz band.
802.11 takes into account the successful and unsuccessful transmission of packets and includes mechanisms designed for dealing with packet transmission problems. For example, 802.11 allows for the retransmission by a transmitter of packets that were not received properly by a receiver.
Transmitters
A typical prior art transmitter 100 is depicted in FIG. 1A. Transmitter 100 includes an encoder 110, a modulator 120, a digital to analog converter (D/A) 130, a radio frequency (RF) front end 140, and an antenna 150, logically interconnected as shown in FIG. 1A.
802.11a wireless communication systems and other wireless communication systems can experience numerous problems during the transmission of signals.
Channel Effects—Fading and Multipath Communication Channels
For example, a wireless communication system could encounter channel effects, such as transmitting signals across a fading communication channel. The fading in the communication channel may be caused by multipath and propagation loss.
In the case of multipath channel, the RF energy that is transmitted between antennas experience destructive and constructive interference due to multiple paths taken by the RF energy with multiple delays on the way to a receive antenna. Such multipath interference modulates the phase and attenuates the amplitude of signals across all frequencies and carriers used by a wireless communication system. In a WLAN, such multipath interference could cause a receiver to receive a packet in error or to miss a packet entirely.
Antenna Diversity
Antenna diversity is a technique used to deal with fading and multipath communication channels. In a wireless communication system with transmit antenna diversity, a transmitter with multiple antennas, is used to transmit signals.
Switch Transmit Diversity
A typical prior art switching diversity transmitter 160 is shown in FIG. 1B. Diversity transmitter 160 includes encoder 110, modulator 120, D/A 130, RF front en 140, antenna switch 142, and multiple antennas 150, 164, logically interconnected as shown.
Prior art diversity transmitter 160 transmits the same information to RF front end 140, which then modulate the information and transmits the same signal via antennas 150, 164, respectively by switching between the two antennas. The downside of this technique is that it provides slow diversity. The antenna switching happens after the transmitter gets to know that the first signal transmission was in error. The delay could cause loss in throughput. The technique also requires a means of feedback from the receiver to the transmitter. Moreover, switching diversity provides limited diversity gain, since only the signal of the selected antenna is used at receiver. Whereas, optimum weighting of the signals transmitted from the antennas would result in greater diversity gain.
Transmit Beamforming
In a wireless communication system with antenna diversity, another way to achieve diversity is with transmit beamforming. With transmit beamforming, the wireless communication system includes a multiple antenna transmitter with transmit beamforming. A typical prior art multiple antenna transmitter with transmit beamforming 170 is shown in FIG. 1C. Multiple antenna transmitter with transmit beamforming 170 includes encoder 110, modulator 120, D/A 130, a transmit beam former 172, and multiple antennas 150, 174, logically interconnected as shown.
Prior art multiple antenna transmitter with transmit beamforming 170 sends information to be transmitted to transmit beam former 172, which then modulates and forms multiple RF signals for transmission on multiple antennas 150, 174, respectively. When modulating and forming the multiple RF signals, transmit beamformer 172, for each antenna 150, 174, weights each RF signal to be transmitted with a complex weight that includes a phase and an amplitude. Such prior art antenna diversity techniques may work well for narrowband signals, where the phase and weights are not frequency dependent. However, the conventional techniques would not work well for wideband signals that have transmitted phase and power that are not constant over the transmitted signal bandwidth and that are frequency dependent, such as 802.11a signals. Therefore, conventional antenna diversity techniques are not applicable to wideband signal wireless communication signals, such as 802.11a signals. Conventional antenna beamforming can not efficiently work for mobile nodes, since the beam can be focused in the wrong direction if the mobile nodes move. In addition environmental effects, such as movement of objects can change the communication channel between the beamforming transmit antennas and the receiver, therefore causing loss of connection.
Space-time Coding
Space-time coding is another way in which a wireless communication system with antenna diversity can encode signals for transmission. Space-time codes, use coding access antennas to achieve diversity gain. The coded signal is transmitted from multiple transmit antennas and is received by multiple receive antennas. A space-time decoder will decode the signal at the receiver. A simple space-time code is a delay-diversity code, where coding is done, by transmitting symbols and delayed replicas of those symbols from two or more antennas.
Such prior art antenna diversity techniques may work well for narrowband signals, where the phase and weights are not frequency dependent. However, the conventional techniques would not work well for wideband signals that have transmitted phase and power that are not constant over the transmitted signal bandwidth and that are frequency dependent, such as 802.11a signals. In addition space-time codes require special decoding processors that are not complaint with the 802.11a standard.
Therefore, a low cost and efficient multiple antenna transmitter beamforming technique is needed that is suited to confront the challenges posed by high data rate wideband packetized wireless communication signals, such as 802.11a signals, that implement frequency dependent weighting in beamforming wideband signals.
Thus, a system and method of low cost and efficient multiple antenna transmitter beamforming of high data rate wideband packetized wireless communication signals is needed.